1. Field of the Invention
The present invention relates to an exposure control apparatus and method for controlling the exposure value onto a photosensitive substrate and, more particularly, to exposure control of an exposure apparatus of a slit-scanning exposure type for exposing a pattern on a mask onto a photosensitive substrate by illuminating a rectangular or arcuated illumination region with light pulses from a pulse light source, and synchronously scanning the mask and the photosensitive substrate with respect to the illumination region.
2. Related Background Art
Conventionally, in the manufacture of a semiconductor element, a liquid-crystal display element, a thin-film magnetic head, or the like using a photolithography technique, a projection exposure apparatus for exposing a pattern on a photomask or a reticle (to be referred to as a xe2x80x9creticlexe2x80x9d hereinafter) onto a photosensitive substrate such as a wafer or glass plate coated with, e.g., a photoresist via a projection optical system is used. Recently, the size of a single chip pattern (one shot area radiated onto a wafer) on a semiconductor element tends to become large, and the projection exposure apparatus is required to expose a pattern having a larger area on a reticle onto a photosensitive substrate (large area requirement). Also, it is required to increase the resolution of the projection optical system in correspondence with a decrease in line width of a pattern of, e.g., a semiconductor element.
However, it is not easy to increase the resolution of the projection optical system and to simultaneously increase the size of an exposure field of the projection optical system. In particular, when a catadioptric system is used as the projection optical system, an aplanatic exposure field often has an arcuated shape.
In order to meet the above-mentioned large area requirement of a pattern to be transferred andlimitation on the exposure field of the projection optical system, a projection exposure apparatus of a slit-scanning exposure type has been developed. In this apparatus, by synchronously scanning a reticle and a photosensitive substrate with respect to, e.g., a rectangular, arcuated, or hexagonal illumination region (to be referred to as a xe2x80x9cslit-shaped illumination regionxe2x80x9d), a pattern, having an area wider than the slit-shaped illumination region, on the reticle is exposed onto the photosensitive substrate. In general, in a projection exposure apparatus, since an appropriate exposure value for a photosensitive material on a photosensitive substrate is determined, the projection exposure apparatus of the slit-scanning exposure type comprises an exposure control apparatus for controlling the exposure value with respect to the photosensitive substrate to coincide with an appropriate exposure value within a predetermined allowable range.
As one technique for increasing the resolution of a pattern to be exposed onto a photosensitive substrate, a technique for decreasing the wavelength of exposure light is known. In association with this technique, of existing light sources, those which emit light having a short wavelength are pulse-oscillation type laser light sources (pulse-oscillation light sources) such as an excimer laser light source, a metal vapor laser light source, and the like. However, unlike a continuous emission type light source such as a mercury lamp, energy of light pulses emitted from a pulse-oscillation light source varies within a predetermined range in units of pulse emissions.
Therefore, in the conventional exposure control apparatus, when the average energy of light pulses emitted from the pulse-oscillation light source is represented by pa, and the range of a variation in pulse energy of the light pulses is represented by xcex94p, it is assumed that a parameter xcex94p/pa representing the variation in pulse energy has a normal distribution (is random). When the number of light pulses radiated onto a certain region (to be referred to as a xe2x80x9cpulse count integrating regionxe2x80x9d hereinafter) on a photosensitive substrate which is scanned relative to an exposure region conjugate with a slit-shaped illumination region illuminated with light pulses is represented by n, by utilizing the fact that a variation in integrated exposure value after the end of exposure is given by (xcex94p/pa)/nxc2xd, the integrated exposure value is controlled to reach an appropriate exposure value within a predetermined allowable range under the assumption that the variation (xcex94p/pa) in pulse energy does not exceed a predetermined value. For example, when xcex94p/pa three times a standard deviation "sgr" is assumed to be 10%, in order to set a desired reproduction precision A of an integrated exposure value three times the standard deviation "sgr" to be 1%, n is 100 or more. Therefore, it suffices if the reticle and the photosensitive substrate are synchronously scanned relative to a slit-shaped illumination region, so that the number of light pulses radiated onto each pulse count integrating region on the photosensitive substrate becomes 100 or more.
However, since conventional exposure value control is open control, when the oscillation state of the pulse-oscillation light source fluctuates for some reason, and the variation (xcex94p/pa) in pulse energy temporarily exceeds 10%, the desired reproduction precision A of the integrated exposure value can no longer be obtained.
In order to solve this problem, in a projection exposure apparatus such as a stepper for exposing a pattern on a reticle onto a photosensitive substrate while the reticle and the photosensitive substrate stand still, as disclosed in commonly assigned Japanese Laid-Open Patent Application No. 63-316430 and U.S. Pat. No. 4,970,546, a modified exposure method for performing exposure by reducing some last light pulses, and a cutoff method for ending exposure when the integrated exposure value reaches an appropriate exposure value within a target precision range are known. In the cutoff method, the number of light pulses radiated onto the photosensitive substrate is not constant. Furthermore, as filed in commonly assigned U.S. patent application Ser. No. 623,176 (""90. 12. 5), a technique for controlling an exposure value by finely adjusting pulse energy in units of pulses is also known.
However, due to the unique feature of the projection exposure apparatus of the slit-scanning exposure type, that is, since light pulses radiated on a plurality of pulse count integrating regions on the photosensitive substrate have different integrated energy levels, the above-mentioned exposure value control method proposed for a non-scanning type exposure apparatus cannot be directly applied.
The present invention has been made in consideration of the above-mentioned problems, and has as its object to provide an exposure control apparatus for an exposure apparatus which synchronously scans a reticle (mask) R and a photosensitive substrate (W) relative to a slit-shaped illumination region by illuminating the slit-shaped illumination region with light pulses, wherein even when a variation in pulse energy in units of light pulses exceeds a predetermined range, an integrated exposure value onto the photosensitive substrate (W) can be controlled to be close to an appropriate exposure value.
A projection exposure apparatus of the present invention which comprises a pulse light source (1) for emitting light pulses whose quantities vary within a predetermined range for every oscillation, an illumination system (2, 5-10) for radiating the light pulses from the pulse light source (1) onto a predetermined illumination region on a mask (R) on which a transfer pattern is formed, and a projection optical system (PL) for projecting an image of the pattern on the mask (R) radiated with the light pulses into a predetermined exposure region on a photosensitive substrate (W), and a scanning system which synchronously scans the mask (R) and the photosensitive substrate (W) upon projection of the image of the pattern, comprises:
(a) a measurement system (14-16) for detecting the intensity of the light pulses radiated onto the photosensitive substrate (W) during scanning of the mask (R) and the photosensitive substrate (W), and measuring an integrated light quantity of each of a plurality of partial regions in the exposure region on the photosensitive substrate (W) on the basis of the detection signal;
the plurality of partial regions being defined by the scanning speed of the photosensitive substrate (W) and the emission interval of the light pulses; and
(b) an adjusting system (16, 19) for adjusting the intensity of the next light pulse to be radiated onto the mask (R) on the basis of a difference between a target integrated light quantity and a measured integrated light quantity of each of the plurality of partial regions upon radiation of some light pulses onto the mask (R).
Also, an exposure apparatus according to the present invention, which comprises a pulse light source (1) for emitting light pulses whose quantities vary within a predetermined range for every oscillation, radiates a plurality of light pulses emitted from the pulse light source (1) onto a first object (R), synchronously scans the first object (R) and a photosensitive second object (W), and exposes a pattern on the first object (R) onto the second object (W), comprises:
(a) an illumination system (2, 5-10) for radiating the light-pulses from the pulse light source (1) onto a predetermined illumination region on the first object (R);
(b) a measurement system (14-16) for detecting the intensity of the light pulses radiated onto the second object (W) during scanning exposure, and measuring an integrated light quantity on each of a plurality of partial regions in the illumination region on the second object, which region is irradiated with the light pulses incident on the second object (W) via the first object (R), on the basis of the detection signal,
the plurality of partial regions being defined by the scanning speed of the second object (W) and the emission interval of the light pulses; and
(c) an adjusting system (16, 19) for adjusting the intensity of the next light pulse to be radiated onto the first object (R) on the basis of a difference between the integrated light quantity and a target integrated light quantity of each of the plurality of partial regions upon radiation of some light pulses onto the first object (R).
As described above, according to the present invention, when a pattern on the mask (R) as the first object is exposed onto the photosensitive substrate (W) as the second object by the slit-scanning exposure method using light pulses from the pulse light source (1), a plurality of partial regions (A1, A2, A3, . . . ) on the photosensitive substrate (W) have different integrated exposure values of the radiated light pulses, as shown in, e.g., FIG. 5. Thus, the measurement system (16,19) detects the intensity of light pulses radiated onto the photosensitive substrate (W), and measures the integrated exposure value so far of each of the partial regions (A1, A2, A3, . . . ) on the basis of the detection signal. The adjusting system (16, 19) calculates a difference between the integrated exposure value so far and a target integrated exposure value to be obtained upon radiation of the next light pulse for each partial region, and adjusts the intensity of light pulses radiated from the pulse light source (1) on the basis of the difference. In this manner, an average integrated light quantity on the entire exposure surface of the photosensitive substrate (W) can be controlled to coincide with an appropriate exposure value within a predetermined allowable range.
Furthermore, an exposure method according to the present invention in which a first object (R) is irradiated with light pulses whose quantities vary within a predetermined range for every oscillation, and the first object (R) and a photosensitive second object (W) are synchronously scanned, and a pattern on the first object (R) is exposed onto the second object (W), comprises the steps of:
detecting the intensity of the light pulses radiated onto the second object (W) during scanning exposure;
measuring an integrated light quantity on each of a plurality of partial regions which are defined on the second object (W) by the scanning speed of the second object (W) and the oscillation interval of the light pulses, and are present within an illumination region of the light pulses which are incident on the second object (W) via the first object (R), when some light pulses are radiated onto the first object (R); and
adjusting the intensity of the next light pulse to be radiated onto the first object (R) on the basis of a difference between the measured integrated light quantity and a target integrated light quantity of each of the plurality of partial regions.
As described above, according to the present invention, the intensity of light pulses radiated onto the second object (W) during scanning exposure is detected, and an integrated exposure value on each of partial regions (A1, A2, A3, . . . ) is measured on the basis of the detection signal, as shown in, e.g., FIG. 5. A difference between the measured integrated light quantity and a target integrated exposure value to be obtained upon radiation of the next light pulse is calculated, and the light quantity of the light pulse radiated from the pulse light source (1) is adjusted based on the difference. Thus, an average integrated light quantity on the entire exposure surface of the second object (W) can be controlled to coincide with an appropriate exposure value within a predetermined allowable range.